Infrared spectroscopy for fuel fraud detection – building reproducible data analysis protocol

Krzysztof Banas

Introduction

• infrared spectroscopy - workhorse for many forensic (and not only) labs

• relatively simple, fast, well-established, inexpensive, safe (no x-rays, isotopes, chemicals)

• limitations: sample thickness, concentration, presence of water

• for fuel research: detection of contaminants, monitoring degradation, quantifying additives

Principles of IR spectroscopy

• most organic molecules absorb light in the infrared region of the electromagnetic spectrum

• absorption at certain wavenumbers corresponds specifically to the bonds present

• every compound has unique spectral signature: absorbance of the infrared light versus the wavenumber

IR modes

\[ Transmittace = \frac{I_0}{I_1} \] \[ Absorbance=-log_{10}T \] \[ Penetration\_ Depth=\frac{\lambda} {2\pi \sqrt{n_1^2 sin^2 \theta - n_2^2)}} \]

Method - Infrared spectroscopy

• various types of samples
• extensive database of spectra
• easily transferable
• specificity and selectivity

• adjusting the sampling method
• volatile samples (repeatability)
• complex chemical fingerprint - many variables
• required data processing

Spectral Ranges

Sampling Techniques

Transmission	Reflection
ATR	Diffuse Reflection (DRIFT)

Transmission

• method of choice, should be used whenever possible
• spectra of the highest quality and adhere to the Beer-Lambert law

\[ A=\alpha*b*c \]

A- absorbance

\(\alpha\) - absorptivity

b - path length

c - concentration

Attenuated Total Reflection

• widely used FTIR sampling technique
• small depth of penetration (path length), virtually “surface” analysis
• relatively easy cleaning
• lower sensitivity than transmission
• response is wavelength dependent
• selection of crystal is required (refractive index and spectral range)

Liquid Samples

Classic Cell	Sealed Cell	ATR
window material variable path length time consuming cleaning	window material constant path length extreme difficult cleaning	crystal material small depth of penetration relatively easy cleaning

Time Trial

Time Trial - Spectra with the shift

Examples of research

ATR-FTIR gasoline	ATR-FTIR Diesel	FTIR Bio-Diesel

Common oil parameters by FTIR

Parameter	Wavenumber [cm-1]
oxidation	1710
nitration	1630
sulfation	1150
Diesel fuel	810
gasoline	750
water	3420
glycol	1080, 1040, 880
soot	2000

Conclusions

• infrared spectroscopy: fast, safe and label-free method for chemical evaluation
• repeatibility and stability is required
• adequate experiment design
• careful spectral data processing especially for quantitative analysis
• both: commercial and R&D approach are important

THANK YOU FOR YOUR ATTENTION

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